Thermochromic Solid-State Emission of Dipyridyl Sulfoxide Cu(I) Complexes

Copper­(I) complexes (<b>Cu-DPSO</b> and <b>Cu-Me-DPSO</b>) utilizing sulfoxide-bridged dipyridyl ligands are reported. <b>Cu-DPSO</b> demonstrates photophysical properties typical with [Cu­(disphosphine)­(diimine)]⁺ complexes, however with additional steric bulk in the 6- and 6′-positions of the diimine ligand to give complex <b>Cu-Me-DPSO</b>; the photophysics are greatly altered. This species is isolated as an amorphous powder (<b><i>a</i>-Cu-Me-DPSO</b>) which emits yellow light; upon heating to 180 °C, a crystalline powder is formed (<b><i>c</i>-Cu-Me-DPSO</b>) which shows a large bathochromic shift (>100 nm) in emission, and shows orange luminescence attributed to a flattening distortion of the complex away from a tetrahedral geometry. On cooling to −196 °C, <b><i>c</i>-Cu-Me-DPSO</b> displays a reversible thermochromic solid-state emission, from orange at room temperature to yellow at low temperatures. Using solid-state variable temperature excitation and absorption data, this phenomenon is attributed to a change in coordination geometry about the copper atom in the excited state. At low temperatures, a pseudo-tetrahedral geometry is preferred, giving higher energy emission, whereas, at higher temperatures, a flattened geometry dominates, giving a lower energy emission under UV irradiation. This is a unique example of a monometallic copper­(I) complex capable of reversible thermochromic emission in the solid state, and highlights the impact that subtle ligand tuning plays on the photophysical properties. Previous copper­(I) thermochromic materials were limited to copper halide clusters, and this study opens new avenues toward highly functionalized, thermal stimuli-responsive materials